1. Field of the Invention
This invention pertains to the general field of optical profilometry. In particular, the invention relates to interferometric measurements conducted through a transmissive medium in the sample and the use of a compensating element in the reference arm of the interferometer.
2. Description of the Prior Art
Interferometric optical systems are widely used to measure surface features because of their speed, accuracy and flexibility. Often the sample surface is tested through a transmissive medium, such as the glass of a biological-sample slide or the package of an electronic device (micro-electro-mechanical systems—MEMS, for example). Similarly, samples placed in an environmental chamber to study the effects of pressure, temperature, humidity and/or reactants are normally tested through a dispersive medium. Also, some parts of devices are operated while immersed in a liquid medium, such as objects used in biology and ink-jet printing, and hard-drive sliders that fly in a lubricating medium. Performing high lateral-resolution metrology on such parts cannot be accomplished using conventional methods.
These transmissive fixed media tend to degrade the interferometric measurement because of dispersion and aberration effects. Moreover, longer working-distance optics are required to accommodate the additional distance to the test surface when such a protective layer is present in the sample. Therefore, when a transmissive fixed layer is present in the path of the measurement beam, an equivalent compensating element in the path of the reference beam of the interferometer has been used in the past to minimize the dispersion, aberration and interference effects of the transmissive layer. While this solution is normally acceptable for low magnification systems (less than about 10×), it has been found to be unacceptable at higher magnifications where all system defects tend to become more and more significant. Even the use of an objective specifically corrected for the aberrations introduced by the transmissive layer was surprisingly found not to improve significantly the quality of the fringes produced by conventional interferometric profilers.
Co-owned U.S. Ser. No. 11/003538 describes an advance in the art of usage of compensating elements by combining such an element with a collimated light and an aberration-corrected objective with a long working distance. This combination was found to produce a greatly improved fringe contrast in the measurement of a sample surface through a dispersive element. When the dispersive element consists of a fixed cover with substantially consistent characteristics from sample to sample, the compensating element is a plate that matches the optical characteristics of the dispersive element. When the dispersive element varies, the compensating element consists of a variable-thickness transmissive element embodied in a pair of half-cube prisms adapted to slide along the beam-splitting plane, thereby permitting the adjustment of the optical path-length through the splitter in the reference-beam direction while retaining unchanged the optical path-length in the test-beam direction.
Inasmuch as the compensating plates described in Ser. No. 11/003538 need to be changed to vary the thickness of the compensating element to match the properties of the dispersive element in the sample, it is crucial that a mechanism be provided that permits the exchange of plates without affecting the calibration and optical performance of the system, or the integrity of both the compensating plate and the optics within the module where the plates are installed.
Another challenge arises from the fact that for many applications the compensating element is very close to the surface of the reference mirror and, therefore, the process of installation or removal can easily cause damage to either or both elements. In order to avoid these problems, prior-art compensating plates were installed as permanent fixtures in the reference arm of the interferometer. Thus, as a further advancement of the concept described in Ser. No. 11/003538, this invention describes a modular housing and a mechanism suitable for the rapid and safe installation and removal of different compensating plates of different thickness.